Psoriatic Arthritis Study of Izokibep: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Psoriatic Arthritis Study of Izokibep (NCT05623345), a randomized phase 2, phase 3 study. The primary outcome is Number of Participants Who Achieved 50% Improvement in American College of Rheumatology (ACR50) at Week 16.

2. Clinical Question

The primary outcome is a count of participants who met the ACR50 response definition at Week 16. The posted results include 18 of 118 participants in the placebo group, 4 of 8 in the 80 mg Q4W group, 49 of 113 in the 160 mg Q2W group, and 45 of 112 in the 160 mg QW group. The main recalculated comparisons focus on the two larger izokibep groups and placebo. The sponsor’s posted analyses also include the 80 mg Q4W group.

The clinical question is therefore about the observed frequency of a defined response at a specified assessment, rather than about change in a continuous score or the timing of an event. It is also a question about these trial groups and this assessment point: the registry record does not establish what the results would be in other populations or under other treatment conditions. Although the registry title describes a placebo-controlled study, the placebo group’s assigned treatment changed after Week 15. The Week 16 primary outcome is assessed at the transition point identified in the group description.

3. Trial Design

The registry describes NCT05623345 as a randomized, double-blind, placebo-controlled, multicenter Phase 2b/3 study of izokibep in subjects with active psoriatic arthritis. The allocation model was parallel, and the record lists enrollment of 351. ACELYRIN Inc. is identified as the sponsor. The study started on 2022-11-21, its primary completion date is listed as 2023-11-30, and its status is terminated.

Four groups appear in the posted primary outcome. Group 1 received placebo from Day 1/Week 0 to Week 15, then izokibep from Week 16 to Week 51. The other groups received izokibep from Day 1/Week 0 through Week 51, at 80 mg Q4W, 160 mg Q2W, or 160 mg QW, respectively. The Week 16 comparison uses the placebo group as the control. The registry record does not explain the circumstances of termination.

The primary endpoint is the number of participants achieving ACR50 at Week 16. The registry defines the primary analysis population as randomized participants who received at least one administration of test material. The posted group counts differ by treatment group, including 8 participants in the 80 mg Q4W group and more than 100 in each of the three other groups. The record does not report why the analysis counts differ from the overall enrollment.

The record lists 10 outcome measures and 21 analyses. It also lists secondary outcomes covering psoriasis response, enthesitis resolution, minimal disease activity, ACR20, PsAID improvement, physical function, and adverse events. The original protocol’s sample-size justification, randomization details beyond the randomized allocation, and further design specifications are not reported in the registry record.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Before assignment: This study was divided into two Periods. In Period 1 (Phase 2b portion of the study), participants received either izokibep or placebo, until Week 16. Period 2 (Phase 3 portion of the study) lasted until Week 52. Participants who received placebo QW during Period 1 started receiving izokibep QW during Period 2 and participants who received izokibep during Period 1 continued to receive izokibep at the same dosing frequency in Period 2.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Full Analysis Set: all participants randomized who received at least one administration of test material was included in the summaries and analyses.

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

Population: Full Analysis Set: all participants randomized who received at least one administration of test material was included in the summaries and analyses.

6. Primary Endpoint

The registry lists the primary outcome as Number of Participants Who Achieved 50% Improvement in American College of Rheumatology (ACR50) at Week 16, measured over Week 16.

The primary endpoint counts participants who achieved ACR50 at Week 16. ACR50 is a binary response: each participant either meets the response definition or does not. The registry description says that meeting ACR50 requires a 50% reduction in tender and swollen joint counts, plus improvement in three of five additional criteria, including pain, global assessments, physical function, and inflammatory markers.

A binary endpoint makes the result directly interpretable as a count of responders in each group. It also allows an absolute comparison between groups: the risk difference expresses how far apart the observed response proportions are. This answers a different question from a relative measure, which describes the response in one group in relation to the other. Both types of summary are useful, but the absolute difference keeps the contrast tied to the observed scale of participants responding.

ACR50 is used as a defined measure of clinical response in this trial, rather than as a measure of every aspect of psoriatic arthritis. A participant’s classification depends on the stated response rule at the assessment point; the endpoint does not by itself describe the full course of disease, the duration of response, or changes after Week 16. Nor does a group-level count show which individuals would have responded under a different assignment. The registry record does not report the detailed procedures for handling missing Week 16 outcomes or the exact computational analysis specification for the endpoint.

7. Results Posted in the Registry

• Izokibep 160 mg Q2W: 43.4% (49/113) versus 15.3% (18/118) on control; risk difference 28.1 percentage points (95% CI 16.9 to 39.3), risk ratio 2.84 (1.77–4.57); unadjusted two-proportion z-test p = <0.001.
• Izokibep 160 mg QW: 40.2% (45/112) versus 15.3% (18/118) on control; risk difference 24.9 percentage points (95% CI 13.8 to 36.1), risk ratio 2.63 (1.63–4.26); unadjusted two-proportion z-test p = <0.001.

8. Secondary Efficacy Endpoints

The registry also posts results for secondary outcomes. For each outcome below, the endpoint type, the test applied to the posted summary statistics, the result and its interpretation are given in turn. Secondary endpoints are usually not adjusted for multiple testing, so they support rather than confirm the primary finding.

Number of Participants With Baseline ≥ 3% Body Surface Area (BSA) Psoriasis Who Achieved a 90% or Greater Reduction in Psoriasis Area and Severity Index (PASI90) at Week 16

• Endpoint type: Binary: the number of participants with the event, compared as proportions.
• Test: Unadjusted two-proportion z-test, with a Wald 95% confidence interval for the risk difference.
• Result:
  • Izokibep 160 mg Q2W: 57.8% (37/64) versus 11.7% (7/60) on control; risk difference 46.1 percentage points (95% CI 31.6 to 60.7), risk ratio 4.96 (2.40–10.25); unadjusted two-proportion z-test p = <0.001.
  • Izokibep 160 mg QW: 64.4% (38/59) versus 11.7% (7/60) on control; risk difference 52.7 percentage points (95% CI 38.1 to 67.4), risk ratio 5.52 (2.68–11.36); unadjusted two-proportion z-test p = <0.001.
• Interpretation:
  • The 95% confidence interval excludes no difference, so the proportion with the event is statistically higher for Izokibep 160 mg Q2W than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.
  • The 95% confidence interval excludes no difference, so the proportion with the event is statistically higher for Izokibep 160 mg QW than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.

Number of Participants With Baseline Enthesitis > 0 With Resolution of Enthesitis (Leeds Enthesitis Index [LEI] = 0) at Week 16

• Endpoint type: Binary: the number of participants with the event, compared as proportions.
• Test: Unadjusted two-proportion z-test, with a Wald 95% confidence interval for the risk difference.
• Result:
  • Izokibep 160 mg Q2W: 55.9% (38/68) versus 46.7% (35/75) on control; risk difference 9.2 percentage points (95% CI -7.1 to 25.5), risk ratio 1.20 (0.87–1.65); unadjusted two-proportion z-test p = 0.271.
  • Izokibep 160 mg QW: 45.5% (30/66) versus 46.7% (35/75) on control; risk difference -1.2 percentage points (95% CI -17.7 to 15.3), risk ratio 0.97 (0.68–1.39); unadjusted two-proportion z-test p = 0.885.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Izokibep 160 mg Q2W and the control. The interval gives the range of effects that remain compatible with the data.
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Izokibep 160 mg QW and the control. The interval gives the range of effects that remain compatible with the data.

Number of Participants Achieving Minimal Disease Activity (MDA) at Week 16

• Endpoint type: Binary: the number of participants with the event, compared as proportions.
• Test: Unadjusted two-proportion z-test, with a Wald 95% confidence interval for the risk difference.
• Result:
  • Izokibep 160 mg Q2W: 41.6% (47/113) versus 14.4% (17/118) on control; risk difference 27.2 percentage points (95% CI 16.1 to 38.3), risk ratio 2.89 (1.77–4.72); unadjusted two-proportion z-test p = <0.001.
  • Izokibep 160 mg QW: 41.1% (46/112) versus 14.4% (17/118) on control; risk difference 26.7 percentage points (95% CI 15.6 to 37.8), risk ratio 2.85 (1.74–4.67); unadjusted two-proportion z-test p = <0.001.
• Interpretation:
  • The 95% confidence interval excludes no difference, so the proportion with the event is statistically higher for Izokibep 160 mg Q2W than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.
  • The 95% confidence interval excludes no difference, so the proportion with the event is statistically higher for Izokibep 160 mg QW than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.

The secondary outcomes cover several domains: skin response among participants with baseline psoriasis, enthesitis resolution, minimal disease activity, ACR20 response, PsAID improvement, physical function measured by HAQ-DI, and adverse events. The posted Week 16 comparisons show that some secondary outcomes have estimates in a direction consistent with the primary ACR50 comparisons, while the enthesitis-resolution comparisons are less conclusive. The secondary measures do not all use the same eligible population: some are restricted to participants with a baseline condition or score, and their analysis counts differ accordingly.

This broad pattern provides context for the primary endpoint, but it is not a substitute for it. Different outcomes capture different aspects of psoriatic arthritis and use different definitions, denominators, and analysis methods. Agreement across some measures can be informative, while divergence is not necessarily contradictory because the outcomes measure distinct constructs and may have different precision. The record lists 10 outcome measures and 21 analyses, so multiple comparisons are part of the evidential context.

Unless a prespecified multiplicity strategy is used, unadjusted secondary p-values carry less confirmatory weight than a single primary test because testing many outcomes can increase the chance of at least one apparently strong result under a null model. The registry record does not report a multiplicity adjustment, testing hierarchy, or which secondary outcomes were designated confirmatory. Accordingly, secondary results are best treated as supportive or exploratory descriptions of the posted outcomes rather than as independent proof of a broad effect across all measured domains. Their interpretation should also account for the different eligible participant groups and the absence of some sponsor analyses for the secondary comparisons.

Secondary outcomes registered for this trial, for reference (only those above were recalculated):

• Number of Participants With Baseline ≥ 3% Body Surface Area (BSA) Psoriasis Who Achieved a 90% or Greater Reduction in Psoriasis Area and Severity Index (PASI90) at Week 16
• Number of Participants With Baseline Enthesitis > 0 With Resolution of Enthesitis (Leeds Enthesitis Index [LEI] = 0) at Week 16
• Number of Participants Achieving Minimal Disease Activity (MDA) at Week 16
• Number of Participants Achieving 20% Improvement in ACR (ACR20) at Week 16
• Number of Participants With Baseline Psoriatic Arthritis Impact of Disease (PsAID) Score ≥ 3 With Improvement in PsAID Score at Week 16
• Change in Physical Function as Assessed by HAQ-DI From Baseline to Week 16
• Number of Participants Who Experienced Treatment-emergent Adverse Events (TEAEs), Serious TEAEs and AE of Special Interest During Period 1
• Number of Participants Who Experienced TEAEs, Serious TEAEs and AE of Special Interest During Period 2

9. Statistical Interpretation

10. Statistical Methods Explained

The primary outcome is binary, so each participant contributes to one of two categories: achieved ACR50 or did not achieve ACR50. The posted results give both the number meeting the endpoint and the number in each analysis group. A group’s response proportion is the basis for comparing the groups, while the risk difference expresses the absolute gap between those proportions. The recalculated risk differences are unadjusted; they are simple comparisons from the posted summary counts rather than estimates adjusted for other variables.

What does the risk difference describe?

The risk difference is the response proportion in an izokibep group minus the response proportion in the placebo group. A positive value means the observed proportion was higher in the izokibep group. For the 160 mg Q2W comparison, the recalculated difference is 0.281; for 160 mg QW, it is 0.249. These are absolute-scale contrasts. They do not indicate that every participant had the same response, and they do not describe a participant’s individual counterfactual outcome under another assignment.

The registry also posts sponsor analyses expressed as risk differences and evaluated using the Cochran-Mantel-Haenszel method. This is a method for comparing outcomes across groups while accounting for stratification, when relevant strata are part of the analysis. The registry record does not identify the strata used, so the adjusted structure of those analyses cannot be described in more detail. The sponsor’s p-values and estimates should be read as the results of that reported method, not as exact duplicates of the unadjusted recalculations.

What does a p-value tell the reader?

A p-value describes how compatible the observed data are with a specified null model, under the assumptions of the test. It is not the probability that the null hypothesis is true, nor does it measure the size or clinical importance of an effect. The recalculated p-values for both larger primary comparisons are <0.001. The sponsor reports p < .0001 for those same comparisons. Those notations reflect the respective analyses as posted and should not be converted into a claim about certainty. The sponsor’s analysis for the 80 mg Q4W group is reported as p = 0.038.

Statistical significance alone does not settle whether an estimate is precise, robust, clinically meaningful, or generalizable. That assessment also depends on the estimate’s scale, its confidence interval, the analysis population, trial conduct, missing data, outcome definitions, and the number of questions tested. Several of those details are not reported in the registry record.

Why can methods yield different estimates?

An unadjusted comparison uses the posted group summaries directly. A Cochran-Mantel-Haenszel analysis can combine information across strata and therefore need not match an unadjusted risk difference exactly. The sponsor’s posted analysis values are 27.69 for the Q2W group and 24.44 for the QW group. The recalculated values are 0.281 and 0.249, respectively. These are not a discrepancy in the registry record: they come from different analysis approaches and are expressed in different numerical formats. The sponsor’s own analysis is the study’s reported inferential result; the recalculations serve as a straightforward, unadjusted companion description.

The posted record does not provide the statistical analysis plan, the randomization stratification factors, covariate adjustments, handling of intercurrent events, or missing-data method. It is therefore not possible to verify whether the reported analysis followed a prespecified plan or to assess the impact of those choices. Nor does the record report any interim analyses or multiplicity adjustment. These absences limit what can be inferred about the intended error control and analysis hierarchy.

How should binary outcomes be understood?

Binary endpoints are easy to summarize as counts, but the observed proportion depends on the participants included and how unobserved outcomes are handled. For an endpoint such as ACR50, a participant with a missing Week 16 assessment cannot be classified from the posted record unless the analysis has a specified rule for that situation. The registry identifies the Full Analysis Set but does not state the missing-outcome rule. General statistical practice recognizes that different missing-data assumptions can affect binary response comparisons; without the trial-specific rule, their influence cannot be quantified.

Over-dispersion is a concern for count outcomes when variability exceeds what a simple count model assumes. It is not the central issue for this endpoint, which is posted as participant counts for a binary response, rather than as a recurrent-event count or continuous measure. The main methodological considerations are the analysis population, group allocation, uncertainty in proportions, comparison method, and treatment of missing assessments. General explanations of these concepts do not establish which procedures were used beyond those named in the registry record.

11. Confidence Intervals and Precision

The confidence intervals for the recalculated primary risk differences describe the statistical precision of the unadjusted contrasts. For 160 mg Q2W versus placebo, the estimated risk difference is 0.281, with a 95% confidence interval from 0.169 to 0.393. For 160 mg QW versus placebo, it is 0.249, with a 95% confidence interval from 0.138 to 0.361. Both intervals are entirely above zero, indicating that the posted unadjusted estimates are compatible with a positive absolute difference across the interval ranges.

A confidence interval reflects sampling uncertainty under the calculation’s assumptions. It is not a range containing every participant’s response, and it does not account automatically for possible bias, missing-data assumptions, analysis choices, or limitations in trial conduct. These intervals are recalculated from posted summary statistics; the registry record does not report confidence intervals for the sponsor’s Cochran-Mantel-Haenszel analyses. The sponsor values therefore should not be assigned these intervals as though they were calculated from that method.

Precision varies with the amount of information in a comparison. The primary group counts for the two larger izokibep groups and placebo are 113, 112, and 118, while the 80 mg Q4W group has 8 participants. Estimates based on a group of 8 can be particularly unstable because a small change in participant outcomes can substantially alter the observed proportion. The registry’s sponsor analysis reports a p-value for that group but no confidence interval.

The intervals quantify uncertainty around an estimate; they do not show that either dose is more effective than the other. The confidence intervals for the two recalculated comparisons overlap, but a direct comparison between the izokibep groups is not posted. The registry record does not report a formal test of those two doses against each other. Precision also does not establish clinical importance, and the record does not state a threshold for interpreting the size of the ACR50 difference as clinically important.

12. Planning a Similar Trial: Sample Size

This is a planning illustration, not a re-evaluation of the original trial. It asks: if a new study wanted to detect a difference like the one observed, or a smaller one, how large would it need to be? It is deliberately not “observed power” of the finished trial, which is not a meaningful quantity.

The planning illustrations use two-sided significance level 0.05, equal allocation between the two arms, and a normal approximation. They use the observed effect as a starting point and show planning under effects set to 100%, 75%, or 50% of that observed effect. These are planning assumptions for illustrations, not the original trial’s design assumptions; the trial’s own sample-size justification is not reported in the registry record.

For the primary endpoint, the figures are participants per arm, not total events. The 80% power column gives the planned number per arm under the stated assumptions for a design with an 80% chance of detecting the selected effect if that effect were true. The 90% power column makes the corresponding planning target more demanding in power and generally requires more participants. These are planning concepts, not guarantees that a future trial will produce a statistically significant result.

For example, planning around 100% of the observed Q2W effect gives 40 participants per arm for 80% power and 53 participants per arm for 90% power. Under 75% of that observed effect, the corresponding figures are 67 and 89; under 50%, they are 137 and 183. The QW illustration gives 50 and 66 participants per arm at 100% of the observed effect, 83 and 110 at 75%, and 170 and 227 at 50%.

Smaller effects are harder to distinguish from random variation, so the illustrations require more participants per arm as the assumed effect becomes smaller. This is not observed power: it is a forward-looking planning calculation based on an observed effect, not a measure of the probability that the completed study detected a true effect. The original design may have used different assumptions, which are not reported in the registry record.

13. How the Sample Size Is Derived

Power is the probability that a study of a given size rejects the null hypothesis when a specific true effect exists. It depends on four things: the effect to detect, the variability of the outcome, the significance level (here a two-sided α of 0.05, so z_1−α/2 = 1.960) and the desired power (z_1−β = 0.842 for 80% and 1.282 for 90%).

Formula

n is the number of participants in each arm, p₀ and p₁ are the control and treatment proportions, and p̄ is their average.

Worked calculation for this trial

1. Control proportion p₀ = 0.153; treatment proportion p₁ = 0.434; average p̄ = 0.293.
2. First term: z_1−α/2 × √[2p̄(1−p̄)] = 1.96 × 0.6437 = 1.2617.
3. Second term: z_1−β × √[p₀(1−p₀) + p₁(1−p₁)] = 0.842 × 0.6123 = 0.5153.
4. n = (1.2617 + 0.5153)² / (0.2811)² = 40.0, rounded up to 40 participants per arm for 80% power.

The smaller-effect rows in the table above repeat the same calculation with the effect multiplied by 0.75 and 0.5 (for ratios, applied to the logarithm of the ratio).

R code

alpha <- 0.05
z_a   <- qnorm(1 - alpha / 2)          # two-sided
p0 <- 0.152542372881356                          # control proportion
p1 <- 0.433628318584071                          # treatment proportion
n_per_arm <- function(power, frac = 1) {
  q1 <- p0 + (p1 - p0) * frac               # smaller effect = fraction of the observed difference
  pbar <- (p0 + q1) / 2
  z_b <- qnorm(power)
  ceiling(round((z_a * sqrt(2 * pbar * (1 - pbar)) + z_b * sqrt(p0 * (1 - p0) + q1 * (1 - q1)))^2 / (q1 - p0)^2, 6))
}
for (f in c(1, 0.75, 0.5))
  cat(sprintf("%3.0f%% of observed effect: n per arm = %s (80%% power), %s (90%% power)\n", 100 * f,
              format(n_per_arm(0.80, f), big.mark = ","), format(n_per_arm(0.90, f), big.mark = ",")))

# Check against R's built-in function (unrounded n per arm)
cat("power.prop.test:", round(power.prop.test(p1 = p0, p2 = p1, power = 0.80)$n, 1), "\n")

# Simulation: how often does the z-test reject at the 80%-power sample size?
set.seed(5623345)
n    <- n_per_arm(0.80)
reps <- 20000
x0 <- rbinom(reps, n, p0); x1 <- rbinom(reps, n, p1)
ph <- (x0 + x1) / (2 * n)
z  <- ((x1 - x0) / n) / sqrt(ph * (1 - ph) * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(z) > z_a, na.rm = TRUE), reps))

Output from running the code

100% of observed effect: n per arm = 40 (80% power), 53 (90% power)
 75% of observed effect: n per arm = 67 (80% power), 89 (90% power)
 50% of observed effect: n per arm = 137 (80% power), 183 (90% power)
power.prop.test: 40 
Simulated power at n = 40 per arm: 0.803 (20000 replicates)

The simulation draws many pairs of event counts at the planned sample size and counts how often the two-proportion z-test rejects the null. It should land close to the target power; at very small required sample sizes it can fall below it, because the normal approximation is rough there.

14. Analyses Reported by the Sponsor

The sponsor reports the primary comparisons using the Cochran-Mantel-Haenszel method. For placebo versus izokibep 160 mg Q2W, the posted risk difference is 27.69 with p < .0001. For placebo versus izokibep 160 mg QW, the posted risk difference is 24.44 with p < .0001. For placebo versus izokibep 80 mg Q4W, the reported risk difference is 36.22 with p = 0.038. No confidence intervals are posted for these sponsor analyses.

The unadjusted recalculations from the group summaries give risk differences of 0.281 for Q2W and 0.249 for QW, each with a 95% confidence interval and p < 0.001. The sponsor’s estimates and recalculations describe the same general primary comparisons, but they arise from different methods and the values are displayed on different numerical scales. A Cochran-Mantel-Haenszel estimate can differ from a direct, unadjusted calculation.

The registry record does not identify the stratification details behind the sponsor’s method, so the difference between the methods cannot be attributed to a specific factor. The sponsor’s analyses are the posted trial analyses; the recalculations add an unadjusted reading of the summary data and do not replace the sponsor’s reported method. The record indicates no discrepancy between the recalculated and sponsor results.

15. Multiplicity, Missing Data and Analysis Populations

The registry record lists 10 outcome measures and 21 analyses. Testing multiple endpoints and treatment comparisons raises a multiplicity question: if each test is interpreted independently without adjustment or a prespecified hierarchy, the chance of at least one low p-value across the family of tests can increase. A multiplicity strategy may allocate error across comparisons, define a sequence of tests, or distinguish confirmatory from exploratory outcomes. The registry record does not report which strategy, if any, was used.

The record includes one primary endpoint at Week 16 and multiple secondary outcomes, but it does not report whether the primary endpoint was tested under a formal hierarchy or whether secondary p-values were adjusted. The sponsor’s analyses identify superiority comparisons and p-values; those entries alone do not establish how multiplicity was controlled. Thus, the posted p-values should be read as results of their stated analyses, without assuming an unreported correction or a prespecified testing order.

The primary analysis population is defined as randomized participants who received at least one administration of test material. This description establishes the stated Full Analysis Set, but does not explain how participants with no Week 16 assessment, treatment discontinuation, or other missing information were handled. The registry record does not report the missing-data method, the number of missing primary endpoint values, or sensitivity analyses for missing outcomes.

Missing-data assumptions matter for a binary endpoint because an unobserved assessment cannot be classified as a response or nonresponse without an analysis rule. Complete-case analysis, imputation, or other approaches can lead to different results, but the record does not identify which approach was used. The registry also does not report the protocol’s original sample-size justification, interim analyses, or full statistical analysis plan. These omissions limit assessment of prespecification, error control, and robustness; they do not by themselves show that any particular procedure was or was not conducted.

16. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Deaths

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 5% reporting threshold.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: angina unstable (12.5%), coronary artery disease (12.5%), supraventricular tachycardia (12.5%), prostate cancer (12.5%), atrial fibrillation (0.9%).

The registry posts adverse-event counts for Period 1 and Period 2 groups. In Period 1, 1 of 118 participants in the placebo QW group had a serious adverse event, and 9 of 118 had another reported adverse event. For izokibep 80 mg Q4W, the counts are 2 of 8 with a serious adverse event and 6 of 8 with another adverse event. In the 160 mg Q2W group, 8 of 113 had a serious adverse event and 78 of 113 had another adverse event. In the 160 mg QW group, 4 of 112 had a serious adverse event and 84 of 112 had another adverse event.

The Period 2 placebo/izokibep 160 mg QW group has 4 of 114 participants with a serious adverse event and 65 of 114 with another adverse event. The registry lists zero deaths in each posted group. Serious adverse events and other adverse events are distinct categories in these counts; the record does not provide enough detail to interpret event severity beyond that distinction.

These are descriptive counts from groups with different exposure periods and group sizes. In particular, placebo participants switched to izokibep after Week 15, so Period 1 and Period 2 do not represent the same treatment contrast. The small 80 mg Q4W group also makes its counts difficult to compare directly with larger groups. The registry record does not provide exposure-adjusted event rates or detailed causal assessments. Counts alone cannot establish whether an event was caused by treatment or support a definitive safety comparison.

17. What the Result Does, and Does Not, Mean

The posted primary outcome shows more Week 16 ACR50 responders in each of the two larger izokibep groups than in the placebo group: 49 of 113 for 160 mg Q2W and 45 of 112 for 160 mg QW, compared with 18 of 118 for placebo. The unadjusted recalculated risk differences are positive, and both confidence intervals exclude zero. The sponsor’s reported analyses also give p-values below .0001 for these two comparisons. Taken together, the posted results indicate a clear statistical separation in this endpoint at this assessment under the analyses reported.

That statement is specific to the trial population, analysis definition, treatment groups, and Week 16 assessment. It is not a conclusion about every symptom, every participant, longer-term outcomes, or treatment effects in people outside the studied population. ACR50 is one defined response measure, and group averages or proportions do not predict an individual participant’s outcome.

The results do not establish that the two larger izokibep regimens differ from each other. The registry record does not post a direct inferential comparison between those izokibep groups. Nor do the results establish the long-term comparison with continued placebo: the placebo group’s assigned treatment changes from Week 16 onward. The termination status and registry information do not explain why the study ended, so no conclusion about the cause or implications of termination is warranted from this record.

Statistical evidence and clinical importance are related but distinct. The observed differences and their uncertainty describe the magnitude and precision of the Week 16 comparisons; the record does not specify a threshold for judging the clinical importance of a particular risk difference. Missing-data procedures, multiplicity handling, and other analysis-plan details are not reported in the registry record. Those limits should remain part of any interpretation alongside the posted estimates.

18. Why This Trial Matters Statistically

This trial illustrates why a binary clinical endpoint should be read through both its counts and its between-group contrast. The number of ACR50 responders is visible in each group, while the risk difference places the contrast on an absolute scale. Reporting the estimate with a confidence interval adds information about precision that a p-value alone does not convey.

It also shows the importance of distinguishing a sponsor’s planned or reported analysis from an unadjusted recalculation of posted summaries. The sponsor used a Cochran-Mantel-Haenszel method, while the recalculated estimates are simple unadjusted comparisons. They tell related but not identical statistical stories, and the method used matters when interpreting the exact numerical estimate and its uncertainty.

Group sizes are another central part of statistical interpretation. The 80 mg Q4W group includes 8 participants, compared with more than 100 in each of the larger primary groups. Its count and sponsor result should therefore be read with particular caution about precision. The planning illustrations further demonstrate, in general terms, that smaller assumed effects require larger planned group sizes under the stated assumptions; they are not evidence of the trial’s original design rationale or of its achieved power.

Finally, the record gives a useful example of how a nominally placebo-controlled design can change treatment assignment over time. The placebo group switches to izokibep at Week 16, so the primary endpoint’s timing and the distinction between Period 1 and Period 2 matter. The record also makes clear how questions about multiplicity, missing outcomes, and safety exposure may remain unanswered when analysis-plan details are not reported. A careful statistical reading keeps these limits alongside the result rather than treating a small p-value as a complete account of the evidence.

19. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

The registry record does not report the reason the study was terminated. It also does not provide the original sample-size justification, the statistical analysis plan, or details of any interim analyses. These omissions prevent assessment of whether the reported analysis choices and planning assumptions were prespecified.

The stated Full Analysis Set includes randomized participants who received at least one administration of test material, but the record does not give a detailed account of missing Week 16 outcomes or their handling. It does not report sensitivity analyses for missing data. As a result, the effect of unobserved assessments on the endpoint cannot be evaluated from the registry entries.

The sponsor’s primary analyses are reported without confidence intervals, and the record does not state the strata underlying the Cochran-Mantel-Haenszel method. The unadjusted recalculated intervals provide one view of uncertainty but are not intervals for the sponsor’s adjusted analyses. A direct test between the two larger izokibep groups is not reported. Multiplicity adjustment for the many outcomes and analyses is also not reported.

The 80 mg Q4W primary group has only 8 participants, making its estimate less stable than those for the larger groups. Secondary outcomes use different eligible subsets, and not all secondary sponsor analyses are posted. Safety counts are shown across treatment periods that do not represent identical assigned treatments, because the placebo group switches to izokibep after Week 15. Exposure-adjusted safety comparisons and detailed causal assessments are not reported. These record-specific limitations restrict how far the findings can be generalized or compared across groups and time periods.

20. Related Tutorials

21. Related Statistical Calculators

22. Sources

• ClinicalTrials.gov: Psoriatic Arthritis Study of Izokibep — NCT05623345.
• Registry record last updated 2025-05-23; status last verified 2024-11.
• Results first posted 2025-05-23.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

23. Record Summary

NCT05623345 is a randomized, double-blind, parallel, placebo-controlled Phase 2b/3 study of izokibep in active psoriatic arthritis, sponsored by ACELYRIN Inc. Enrollment is listed as 351; the record lists a start date of 2022-11-21, primary completion of 2023-11-30, and terminated status.

The primary Week 16 endpoint is ACR50, a binary response. The posted counts are 18 of 118 for placebo, 4 of 8 for izokibep 80 mg Q4W, 49 of 113 for 160 mg Q2W, and 45 of 112 for 160 mg QW. Recalculated unadjusted comparisons for the two larger izokibep groups have positive risk differences with confidence intervals excluding zero; sponsor analyses report p < .0001 for both comparisons.

The registry also lists secondary efficacy and safety outcomes. The trial’s original sample-size justification, detailed missing-data procedures, multiplicity approach, and reason for termination are not reported in the registry record.